Field of the Invention
Transfer materials are stuck to an image substrate, for example, after printed using an ink jet printing system, so as to be used for labels, ID cards, packaging materials, building materials, and other various applications.
Description of the Related Art
In the ink jet printing system, an ink receiving layer of a transfer material needs to absorb a large amount of ink in order to achieve a sufficient image density. Examples of the ink receiving layer include a swelling absorbing type mainly formed of water-soluble resin and containing ink in a network structure of a water-soluble polymer and a gap absorbing type that contains ink in a fine gap structure. The gap-absorbing ink receiving layer is preferably used because a large amount of ink can be absorbed into air gaps in the ink receiving layer. However, when an ink receiving layer surface is appropriately attached to an image substrate after ink jet printing, while ink absorbability is maintained so as to absorb a large amount of ink, specific problems may occur which are attributed to the ink receiving layer that can absorb a large amount of ink.
For example, the ink receiving layer surface may be attached to the image substrate by bonding particles together with resin to bring a gap-absorbing ink receiving layer with air gaps formed therein into which the ink is absorbed into close contact with the image substrate and heating the resultant laminate to a temperature higher than a glass transition temperature Tg (dissolution temperature) of the resin, which serves as a binder. In this case, the problems (1) and (2) may occur.
The ink receiving layer surface is insufficiently smooth, and the amount of resin serving as a binder for particles is insufficient to cover the entire ink receiving layer surface, making adhesion to the ink receiving layer difficult.
The resin serving as a binder for particles has a weak affinity to the material of the image substrate depending on the combination of the resin and the material, making adhesion difficult.
First, the problem (1) will be described. The gap-absorbing ink receiving layer has spaces resulting from bonding of particles with the resin and serving as air gaps into which the ink is absorbed, and can thus absorb a large amount of ink into the air gaps. However, a countless number of recesses and protrusions formed of exposed particles are present on a surface of the ink receiving layer. In a common configuration of the gap-absorbing ink receiving layer, the number of resin components functioning as a binder is substantially smaller than the number of particles, and thus, a large number of air gaps are formed to provide sufficient ink absorbability, resulting in enhanced ink absorbability during ink jet printing. After ink jet printing, the ink receiving layer may be attached to the image substrate by being brought into close contact with the image substrate and heated so that the resin components functioning as a binder is dissolved at a temperature higher than Tg (dissolution temperature) and flow and come into contact with the image substrate. A countless number of recesses and protrusions formed of exposed inorganic particulates are present on the surface of the ink receiving layer formed by adding together approximately 90% inorganic particulates and approximately 10% water-soluble resin functioning as a binder that binds the inorganic particulates together. When the surface of the ink receiving layer is attached to the image substrate, even though the water-soluble resin is heated to a temperature equal to or higher than the glass transition temperature and dissolved and flows, only a small amount of flowing water-soluble resin comes into contact with the image substrate. Thus, it may be difficult to sufficiently fill, with the dissolved water-soluble resin, the space between the surface of the ink receiving layer with the countless number of recesses and protrusions formed of non-adhesive inorganic particulates and the image substrate surface, resulting in inappropriate adhesion. Increasing the amount of water-soluble resin allows adhesion to be strengthened. However, the air gaps between the inorganic particulates are likely to be filled, degrading the ink absorbability during ink jet printing to preclude appropriate image printing characteristics from being achieved.
Now, the problem (2) will be described. To allow the ink receiving layer to appropriately adhere to the image substrate, materials having an affinity to each other need to be selected for the image substrate and the resin components of the ink receiving layer. When the resin components and the image substrate are dissolved by heat at the time of adhesion, the affinity between the resin components and the image substrate is enhanced. The resin components are firmly attached to the image substrate by an intermolecular force between the component material of the resin components and the component material of the image substrate. However, in many cases, the material of the image substrate and the resin components of the ink receiving layer may have a low affinity to each other depending on the combination of the resin components and the material of the image substrate. Thus, when the gap-absorbing ink receiving layer is attached to the image substrate, the ink receiving layer fails to be attached to the image substrate depending on the combination of the ink receiving layer and the material of the image substrate, and the material of the image substrate for attachment is limited.
Thus, if the ink receiving layer and the image substrate fail to adhere to each other, a highly adhesive primer layer needs to be provided between the ink receiving layer and the image substrate. Thus, the ink receiving layer and the image substrate need to be attached to each other via the primer layer. However, providing the primer layer needs a separate step of forming the primer layer after image printing. Thus, disadvantageously, a relevant apparatus has an increased size, and a transfer speed is reduced and thus limited because the primer layer is generally formed by thermal transfer.
Consequently, a technique has been proposed in which an image printed using an ink jet printing system is attached to the image substrate (transfer target material) without the use of a primer.
For example, Japanese Patent Laid-Open No. H09-240196 (1997) describes a transfer image forming sheet material including a porous adhesive layer and an ink receiving layer formed under the adhesive layer. The ink receiving layer receives and fixes the ink from an ink jet printing apparatus via the porous adhesive layer, and is configured to absorb the ink transmitted through the porous adhesive layer.
Japanese Patent Laid-Open No. 2013-39791 describes a transfer film including an ink permeation layer having air gaps through which the ink infiltrates and an ink receiving layer allowing reception of the ink having passed through the ink permeation layer. The ink permeation layer is charged to have the same polarity as that of the ink so as to promote permeation of the ink through the air gaps, and the ink receiving layer is charged to have the polarity opposite to the polarity of a color material in the ink. The ink is absorbed into the ink receiving layer through the ink permeation layer.
In Japanese Patent Laid-Open No. H09-240196 (1997), a swelling absorbing ink receiving layer is used. Upon absorbing the ink, the swelling absorbing ink receiving layer partly swells and becomes non-smooth. When an ink receiving layer having a surface that is non-smooth and that is uneven is attached to an image substrate, the unevenness of the surface weakens the adhesion between a transfer film and the image substrate, possibly making the adhesion between the image substrate and the ink receiving layer difficult. To reduce the adverse effect of the unevenness of the surface of the swollen ink receiving layer, the adhesive layer may be made thicker. However, an increased thickness of the adhesive layer leads to the need for a long time to allow the ink to pass through the ink permeation layer. Then, the ink stays in the adhesive layer for an increased length of time, spreading ink dots that form an image to make the image likely to bleed. To smooth the uneven surface of the swollen ink receiving layer, the ink receiving layer may be sufficiently dried before being attached to the image substrate. However, a long time is needed to sufficiently dry the ink receiving layer, disadvantageously limiting the transfer speed. A separate dryer may be provided to promote drying of the swollen ink receiving layer to smoothen the uneven surface. However, this disadvantageously leads to an increased size of the apparatus.
Furthermore, the porous adhesive layer has the property of allowing permeation of the ink by capillary action and thus absorbs the ink at high speed. On the other hand, the swelling absorbing ink receiving layer mainly formed of water-soluble resin and containing the ink in the network structure of a water-soluble polymer needs a long time to absorb the ink. That is, an ink absorption speed of the porous adhesive layer is much higher than an ink absorption speed of the swelling absorbing ink receiving layer. Thus, ink droplets having landed on the porous adhesive layer are quickly transmitted through the adhesive layer to reach an interface between the adhesive layer and the ink receiving layer. However, since the swelling absorbing ink receiving layer absorbs the ink at low speed, the ink may stagnate in the adhesive layer on the ink receiving layer surface. As a result, the ink dots that form an image spread, leading to the likelihood of image bleeding and a decrease in resolution.
Moreover, the swelling absorbing ink receiving layer absorbs the ink at low speed and thus fails to instantaneously absorb a large amount of ink. Thus, a large amount of unabsorbed ink having failed to be absorbed by the ink receiving layer remains in the adhesive layer after ink jet printing. If, in this state, an attempt is made to attach the adhesive layer onto image substrate by bringing the adhesive layer into close contact with the image substrate, the unabsorbed ink flows back to the surface of the porous adhesive layer to cover the area between the adhesive layer and the image substrate, leading to inappropriate adhesion. Furthermore, moisture remaining inside the porous adhesive layer may rapidly vaporize during thermal transfer to form voids, resulting in inappropriate adhesion. When the ink is sufficiently dried so as not to hinder adhesiveness, the speed of ink jet printing may be significantly reduced. Maintaining the appropriate printing speed needs a special drying unit used after ink jet drying, resulting in an increase in the size of the apparatus and complication of the apparatus.
In Japanese Patent Laid-Open No. 2013-39791, the adhesive ink permeation layer has air gaps through which the ink permeates, and ink jet printing is performed on the ink permeation layer side to allow the ink having passed through the ink permeation layer to be contained and absorbed into the gaps between ink receiving particles in the ink receiving layer. However, in the air gaps in the ink permeation layer, the ink may aggregate, and thus, it is difficult to allow all of the ink having landed on the ink permeation layer to uniformly pass through. Thus, the ink remaining in the air gaps in the ink permeation layer in an isolated manner may flow back to the surface of the ink permeation layer during ink attachment, leading to inappropriate adhesion.
Thus, in Japanese Patent Laid-Open No. 2013-39791, the ink permeation layer is charged to have the same polarity as that of the ink so as to prevent aggregation of the ink in the air gaps in the ink permeation layer, whereas the ink receiving layer is charged to have the polarity opposite to the polarity of the ink so as to allow the ink to be absorbed into the ink receiving layer instead of remaining in the ink permeation layer. However, a relatively high electric force is needed to shift all of the ink, absorbed into the air gaps in the ink permeation layer by a strong capillary force, to the ink receiving layer side based on the difference in charging polarity. During a process in which the ink infiltrates through the gaps in the ink permeation layer, a portion of the ink separated and isolated from the remaining portion of the ink by some of the air gaps remains stagnant in the air gaps. Consequently, preventing the ink from remaining in the ink permeation layer is difficult.
Thus, in Japanese Patent Laid-Open No. 2013-39791, an ink permeation liquid that allows permeation of the ink to be promoted is ejected using the ink jet printing system to push the ink from the ink permeation layer to the ink receiving layer. However, a separate mechanism that ejects the ink permeation liquid needs to be provided, disadvantageously leading to an increased size of the apparatus. Thus, this method lacks practicality.
As described above, image bleeding or a decrease in printing resolution may occur in a configuration in which an adhesive ink permeation layer is provided all over the surface of the transfer material so as to absorb the ink into the ink receiving layer through the ink permeation layer. Moreover, the ink may remain on the surface of the ink permeation layer or inside the ink permeation layer to cause inappropriate adhesion. Thus, achieving both appropriate ink jet printing characteristics and appropriate adhesiveness is difficult.